Yes, you can move things around, but not too much. At least in my case, I installed all the electronics and batts at the rear bay and the ign. batt over the fuel tank but I had to add some tail weight anyway. I don´t see what else to do for balancing the plane without weight, but is not that big deal at all. It floats at slow speed.
Sewerdude, don´t think that hard and get the reactor!!! . You will enjoy it.